Boulder close-up

The scientific imaging system OSIRIS on board ESA’s spacecraft Rosetta has caught a spectacular glimpse of one of the many boulders that cover the surface of comet 67P/Churyumov-Gerasimenko.

Boulder Cheops, taken by Rosetta’s OSIRIS narrow-angle camera on 19 September 2014, from a distance of 28.5 km. The boulder has a maximum dimension of about 45 m. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

With a maximum dimension of approximately 45 metres, it is one of the larger boulders seen on the comet. It stands out among a group of boulders in the smooth region located on the lower side of 67P/C-G’s larger lobe.

This cluster of boulders reminded scientists of the famous pyramids at Giza near Cairo in Egypt, and thus it has been named Cheops for the largest of those pyramids, the Great Pyramid, which was built as a tomb for the pharaoh Cheops (also known as Kheops or Khufu) around 2550 BC.

This choice also introduces a more general Egyptian naming scheme agreed by Rosetta scientists that will be used for many of the features on 67P/C-G, in keeping with the spirit of the overall mission and spacecraft names.

Cheops was seen for the first time in images obtained in early August upon Rosetta’s arrival at the comet (see below –can you spot it?). In the past few weeks, as Rosetta has navigated closer and closer to the comet, OSIRIS imaged the unique structure again – but this time with a much higher resolution of 50 centimetres per pixel.

The lower side of 67P/C-G’s larger lobe. The image was presented on the occasion of arrival on 6 Aug; it was taken from a distance of 130 km and the image resolution is 2.4 metres per pixel. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The boulder-like structures that Rosetta has revealed in many places on the surface of 67P/C-G are one of the comet’s most striking and mysterious features (see yesterday’s CometWatch for a focus on the boulders on the comet’s ‘neck’ region).

Just like the many other boulders seen by both OSIRIS and the spacecraft’s NAVCAM, Cheops stands out not just physically, but also as a slightly brighter feature compared to the darker surface around it.

OSIRIS Principal Investigator Holger Sierks, from the Max Planck Institute for Solar System Research (MPS) in Germany, describes the surface of Cheops as “very craggy and irregular.”

Interspersed between the lighter lumps on the boulder’s surface are intriguing small patches of darker material, similar in brightness and texture to the ground upon which the boulder lies.

“It almost looks as if loose dust covering the surface of the comet has settled in the boulder’s cracks. But, of course, it is much too early to be sure,” says Sierks.

Apart from their size distribution, which is being measured through careful analysis of the images, almost all other properties of 67P/C-G’s boulders are still a mystery to researchers. What are they made of? What are their physical properties, including density and stability? How were they created? As Rosetta continues to survey and monitor the comet’s surface in the next months, the scientists will be looking for clues.

“For example, if the boulders are exposed by cometary activity or are displaced following the comet’s gravity field, we should be able to track this down in our images,” adds Sierks.

Comments

135 Comments

And to imagine it is still at a distance of 28.5 kilometers. Tomorrow ESA will begin their Close Observation Phase at 10 Kilometers distance. That is almost 3 times as close which means a further enormous boost in resolution and detail.

Just one of the most stunning space science missions in history. And if the lander can land. Even drill into the surface to see possible building blocks for life. Or the same water signature as on Earth. Or the comet tail development over the next 12 months seen from the orbiter. Plumes visible erupting from the surface, etc. Interior mapping using the lander and the orbiter.

Dust sample analysis also on the orbiter. So much coming up. Though landing on this impossible object will be extremely high risk and a large chance of failure. Especially since it is the first attempt in history and many first attempts don`t work out well. But lest hope for the very best. And for more of these close up stunning images.

The full frame size is about one kilometer each side and the pixels are about a quarter of a square meter each of the 4 mega pixels. Just great, if Philae would have bin there it would be visual as it would cover at least 4 pixels and those would be very shiny due to its much better reflectivity, also having a long shadow standing on its feet.

On downloading the image it is 1,567 x 1,066 pixels, so about 788 x 533 metres. This is therefore a cropped OSIRIS image. The rest of the image obviously revealed too much of interest. No mind, it is very nice to see an OSIRIS image of the surface.

Surprising is that the surface is everything but smooth and joyful to see is that the boulder has some dust pockets as well. I think this dust is not very fluffy but rather dense and i do not longer fear that the Philae will sink too deep into it. Most likely the animation of it fits the bill. Super!

If you made a replica of this scene on earth is would be a challenge to mountain bike it or eve take a stroll across it. The 55 meter long almost vertical scar at the top left corner is interesting and we well see what comments i will create. The pitch black round small hole is not a vent hole but a tiny deep crater for sure.

If assuming that the solar angular hight over the horizon is about 30 to 45 degree then the smoothest horizontal flats on this place has a valley to peak hight of at least 1.5 meters for every 3 meters of length, thats a ruff terrain, don’t call that a smoothie and to extrapolate it the small scale texture of the surface would make anyone to stumble. Its odd how well the artistic picture of Philae parking on the comet fits the bill.

Thank you! To me this “smooth” area seems to be covered with rather coarse and heterogenous material (differences in reflectivity) the “wind swept” character might be created during outgassing, or during reimpact: obviously a gravity based sedimentation. And likely that finer material escaped to form the coma… but the “rock”? We want a rover! Start building a new rocket…

I agree Jacob, the surface looks “coarser” than I would have expected. The pictures from Curiosity show the very fine sand/dust on the totally dry surface of Mars clump quite easily into larger “balls”. If there is indeed a component of the surface that is “tar” like, very fine dust may clump into even larger objects, pebble size.

I think there is evidence that Cheops has moved. There is a “bow wave” of material “above” the rock and behind it evidence of a shallow trench maybe 80-100m long. The movement appears to have been stopped as it toppled over the edge of a depression marked by the lighter markings to the left and right of Cheops. On the larger scale picture the whole depression is more obvious.

There is a clear indentation to the right of Cheops as if created by sand in the lee of the prevailing wind. There is no wind on 67P. The dust falling to the surface from the coma would be falling very slowly so the rotation of the comet would mean Cheops intercepts dust as it moves through the falling dust, creating a “dust shadow”. It is similar to driving in rain. There is a proper scientific name for this effect which escapes me at the moment.

It is also more obvious from this image that, the undulation and features seen on these smoother plains is due to the topology of the more solid features beneath the surface layer. This is best seen in the crescent shaped scarp at the left of the image. This scarp is about 2-3m high and has not been buried yet by material falling onto the surface. The large 60-70m “scar” top left appears to be lines of smaller “rocks” 1-2m across which are only partially buried. I presume the science team can make estimates of the depth of this “regolith” from such images.

Let us assume the darker “smooth” areas are darker, because activity to erode this layer is less and exposure to radiation, thought to be creating the charcoal colour surface, has been acting longer. Lighter areas, such as we see in the neck region and the “flat” area adjacent to the cross marking the centre of the landing ellipse, (labelled G in Bill’s topographic map), which I shall call Cleopatra because of its milky appearance, would be areas with higher surface layer erosion rates. The conclusion being the “regolith” layer in lighter areas is not as deep as in darker areas.

Cleopatra is about the size of a football field, perhaps a bit bigger. I am hoping Philae can land here, with what might become an active vent in the middle, scarps and cliffs with scree and rubble at the bottom of them and a couple of larger boulders, all within view of Philae’s panoramic cameras. It is also “flat”, but shows some evidence of sub surface features made of virgin material that might be accessible to Philae’s drill.

On closer inspection, I am thinking more in terms of a giant mogul field. With lumps and bumps creating variable amounts of shadow to give the coarse appearance from distance. The moguls maybe only a metre or less high, and not steep sided and close up, the surface quite smooth.

The rim of the little crater/pit is quite well defined so the material does seem to have some cohesion, otherwise a dry, dusty, loose surface would have fallen in and partially filled it leaving a very indistinct rim.

Arctic ice sheets on earth accumulate dust and soot particles and between snowfalls actually appear rather black (not C-G deep black). The surface of so called ablative ice, often have a varying appearance of “wave like” or “mogul field” patterns (help me glaciologists!). With current Osiris 45 cm resolution it is wide open to interpretation: rocky, coarse? or: smooth, “mogul field”?. However: if the surface is a porous, dry black blanket covering a sheet of ice, the “undulations” may be patterns caused by solar heating creating self-amplifying depressions where different orientations receive different radiation energy: a common phenomenon. The apparent size of the pits in this “mogul field” seems to be in the 1 meter range, which in this interpretation will indicate that the thickness of the “black sheet” is somewhat less than 1 metre: a mere 20 cm or less would be my guess. Only a suggestion.

Pretty much my thinking Jacob. Moguls are actually a man made phenomenon, but I used the term to give the idea of a lumpy, bumpy appearance, created as the dust surface layer covers rubble and unevenness in the terrain, from “eroded” small craters, cracks and fissures, small, once active vent structures and the depressions you describe.

I think because of the low gravity and some cohesiveness from the organics in the surface layer, the bumps might be quite steep sided and a lot more randomly shaped than round snow moguls.

I have seen no definitive evidence of dust slides, not even ones created by falling rocks. With such a low gravity and material density, there is not much stored potential energy in falling or rolling objects. Difficult to get your head round everything moving SO slowly on 67P.

@Robin Sherman. We are discussing the relative un-smoothness, but really this area is uncredibly smooth, and probably level (equal gravity). At least in some stage in the past, the material (or layered materials) behaved like a liquid, and made it possible for the ultra weak gravity to level things out. During that stage, which might be ongoing, other forces that would disrupt this process must be even weaker. That is: no impacting, not even slooow. Comet quakes, on the other hand, may help the leveling process: very fine powder become “liquid” when stirred. This goes for the likes of cement or, preferably, some qualities of carbon black (known as laser printer toner or the black in tyre rubber). With my limited knowledge I find it hard to come up with a chemical composition of a possible liquid (or former liquid). It would have to have an absolutely amorphous structure, iIn order to achieve this overall flat. Something like molten pitch maybe? That would have required high temperatures, that in turn would require a “dry” base to rest on, in order not to be blasted away (molten pitch on dry ice. Don’t try this at home, or in space) . I can’t reject any liquid, but I cant imagine the scenario. Then for me it would have to be the printer toner. Poor Philae! I hope it lands on a cinder on does not get too clogged up with (my imaginative) sooty dust.

These large dust covered areas are amazingly even in relative terms. I would agree, that to create such a surface the most likely candidate would be a liquid. Some of these “flat” areas are hundreds of metres across. Thats a lot of liquid to even out the underlying topology, the rough, scarred and rubble strewn areas we see elsewhere.

My initial theory was liquid produced by the heat and pressure of impacts. Many of these apparent impact craters would require very high kinetic energy objects. Given the low density, brittle and porous nature of the comet, as others have pointed out, this number of such impacts would surely have broken the comet into fragments.

The only other source of fluid In such large amounts I can think of is from some type of volcanic process. This would require large amounts of heat to be generated internally, several orders of magnitude greater than the current tidal energy created by the head and body. As the comet approaches the sun those tidal forces will increase. By how much I don’t know.

Given the “erosion”, of what I shall now call caldera walls, those events must have taken place a long, long time ago, perhaps due to the heat generated during the original accretion. I am not sure how far back in time the orbit of 67P is known, but some Jupiter class comets are known to have been trapped in orbit around Jupiter at times in their history. If this was the case, tidal heating, such as we see on Io, could have provided enough energy for Cryovolcanism.

Having established a level surface with respect to the gravity field and then frozen, deposition on the surface of the fine dust, toner ink is also an analogy used by Rosetta scientists, would reflect the contours of the surface beneath as it evolved. It must have taken thousands if not millions of years for that dust layer to build up and during that time I am sure some sublimation of the solid icy layer would still have taken place, with results similar to sea ice and glacier surfaces.

Close up, on a millimetre scale, such a fine dust would surely give a very smooth appearance to the top surface. Comeatstalker suggests the Philae landing video is a pretty good representation of the surface and I would tend to agree, though maybe not so spiky. How deep Phiae will sink into the dust is probably unknowable prior to landing, even if instruments like ALICE can characterise the chemistry of the surface material. It probably varies considerably in depth from place to place. How much resilience or stickiness is added by “tar” like organics is also a factor.

It is straightforward to see this as a rock on a seafloor… but where did the rock come from? Below? Does it float? Is it a low density crystalline and porous material? (A snowball floating in a pond of dirty pitch?)

Its hilarious how well the two images are aligned. Its as it is just a zooming but it actually is a lot of orbital changes and time between them. Im sure this is not done by chance but on purpose. Great, it would be possible to do a very nice rendering, zooming into the boulder from far outside the comet. In my imaginative film-camera i can see it clearly.

The boulder has a mass of about 16 metric tons if its density is as the average of the comet, the weight it has on the comet is as if you would hold a mass of 0.26 kg in your hand, just about a paper mug of water. To realize that it has been sitting there for a huge ammount of time, possibly long before mankind left their caves just waiting for us to evolve and finaly look down on it is the most amazing thing about this mission to me.

Comeatstalker, to me it looks like a rock. If it had the alleged density of the comet as a whole, it would in all logic bob like a champagne cork on any pond, lake, sea or ocean down here on Earth.

Personally, I prefer to believe what I see. (I also thought that was precisely why we sent Rosetta to take a close look at this comet in the first place, and equipped it with all those hi-tech cameras). The shock for theoretical expectations has been total, and will increasingly be so as Rosetta sends back increasingly precise images and data.

I can only advise you to trust what your eyes are telling you: down here on Earth, this boulder would immediately sink like a stone to the bottom of the ocean (along with the rest of the comet and the standard “dirty snowball” model with it).

Just a quick math check, if the boulder were spherical, with a diameter of 45 meters and a density of 0.4 g/cm^3, it would have a mass of 19,000 metric tons. Since it is not spherical, we can probably half that for a rough estimate, but it is certainly not 16 mt. For a mass of 10,000 mt, I get a “weight” of ~75 kg, so you need a couple people to lift it 🙂 A couple people and a LOT of patience, it might not be that heavy, but its still massive.

I once had a sailboat that weighted 7 tons and to move it by hand in harbor was quite easy, the inertia 10 to 20 tons gives is not a big deal to get moving if the friction is low. To compare i hardly can move my 2.5 ton car by hand. If this boulder was an iceberg it would be possible to push it a bit and once it starts to move its keeping its path for quite a while. Now if my calculator is a bit bugged or not i did not check but still it is a low density and gravity and i dont think this rock did roll a lot or emerged or is sinking . I do believe what i see and i see a lot of really large craters and as craters go they are a result of a lot of trashing due to some impacts, some of the material regathered at secondary an so on impacts. I say this rock fell down for a long time ago and is still sitting there. I also think that this binary shape was a result of a really friendly almost kissing impact resulting in the neck as the two bodies filled up their Roche lobes with its brittle material creating the neck. If very high energy impacts would ever have occurred on this comet it would be scattered to dust.

I don’t calculate too much as there is no input data available, just the result is there and to reverse its creation is too hard to do by calculus. I think before this comet altered its orbit to be in reach for us it was in a quite and nice place with friendly neighbours orbiting each other.
Several of those lumps got a bit closer and collided but the impacts where not too volatile and most of them added up in bigger lumps still orbitig each other, Finally two where still neighbours in close orbit and decided to get even closer as it sometime happens. Close orbiting bodies adapt their rotational angular speed to a common one. At some point they got in contact. Some collisions with other bodies pushed them out of their community and with a bit help from large planets like jupiter it got within our reach.
The time scale can be anything from billions to millions of years and its composition has a low average density wich doesn’t mean that it is of uniform composition. The next comet we will get close to might be just as this one or totally different. Even if comets seem to be complex they are very simple in comparison to what molecules that are a result of communities of atoms are able to create. The search for the knowledge of how our solar system was created is not very important nor is the search for the creation recipe of comets. The most importent thing is to develop the skills off mankind and this mission is a tiny bit in this process that so far has been going on for just about no time at all. No how big are the chances that an Ape creature after a few million years ads some text in this blog? I say 100.0%

Wonderful. These boulder images are as good as geological road-cuts and give a great deal of insight to the accretion processes.

My favorite rock is the BIG one seen at the top of the Site J images. I’ve named it “Bulwinkle”. 🙂

And the higher resolution images are giving a better understanding of the surface character. My underlying concern is that the surface is covered with nano- and micron-sized particles, un-consoilidated (-compacted) and Philae will sink down past her knees. Same fears that were experienced with the first Moon landers in the ’60’s.

Bill, may I ask precisely what insight this boulder is giving us into your hypothesized “accretion processes”? To me, it just looks like a rather large lump of rock which was probably dislodged from one of the surrounding, equally rocky cliffs, (like the other, nearby boulders) in an indeterminate past.

I guess we’re all going to be seeing what we want to until the scientific evidence is finally delivered and settles things one way or the other.

Have call it something else though. The Sphynx maybe.
It is very lonely sat in the deserted plain there. Looks eerily similar to Cheops. No idea how either of them got where they are. Differential erosion must have something to do with it.

I’m thinking there are enough organic hydrocarbons, “tar”, to bind the dusty regolith together to make it strong enough for Philae not to sink too far into it. She does only weigh 1g at the surface. As long as the momentum absorbing gadget works, she’ll be fine. It might even be “sticky” enough to stop her bouncing.

The caption asks whats it made of?
How would we know?, you guys have the instruments. It there is no watrer ice on these large boulders that have presumeably fallen and rolled from some sort of cliff.
Then where can you go with the dirty snowball theory?
If there is no ice on this and it has broken off an exposed surface, then how deep do we need to dig to find ICE ?

“It’s absolutely astounding,” says Karl Battams of the Naval Research Lab in Washington DC. “I did not think the comet’s icy core was big enough to survive plunging through the several million degree solar corona for close to an hour, but Comet Lovejoy is still with us.”

I don’t want to sound too pessimistic but personally, I think that, paradoxically, the “dirty snowball” myth doesn’t stand a proverbial “snowball in Hell’s chance” of being eradicated that easily, even if it were to be utterly falsified (again) by the findings from Rosetta and Philae, as it already was by Comet Lovejoy (not to mention by the Deep Impact mission).

You have to understand that there’s too much at stake out there, Dave.

About the OSIRIS photo — the caption says the boulder is 45 meters wide, and that the image has a scale of 50 cm/pixel. But the boulder is about 200 pixels wide in the enlarged photo. Has the OSIRIS image been enlarged by a factor of two?

@simon. Until we see them actually rolling around, they could be protrusions of much larger internal structures, like tips of “icebergs” but we know there is no ice at the surface. I do not think that they are boulders, but I can accept boulder-like.

The Boulder, perhaps its been there a long time but what is its origins?
Perhaps its a boulder that was drawn into the Gravitational field of the Comet many ages ago, that is one likely source.
Then again it could have impacted the Comet many ages ago as the Comet wanders among the Planets.
The surface area looks very much like sediment which has come down very slowly from the ejecta. You could simulate that with flour and a sieve.
Remember all that is happening on the Comet has no time line and has been happening continuously maybe since time began.
The image from the OSIRIS camera is very welcome and I look forward to seeing the next allowed image.

The Boulder is gravity sufficient; certainly upon the Orbit of Lighter G-mol. Its makeup is probably in, and that of the same. The movement of the Boulder upon the Comet Surface is indicative of a ‘Once upon a Time’ movement; probably a Spacial Shift expounding from Galactic Spiral Arm. The dust, and depth of it ‘Track Covering’ can help Theory based upon in my opinion; “Euncleadus” of Saturn. Definetly with Saturn Solstice, and such Activities thereof. #GoldAU

Regarding the origin of bulders: my preferred hypothesis is that they are giant hail composed of ejecta containing water and/ or other material that will crystallize in space. The timing of the formation of such giant hail (as well as most) is beyond my knowledge. Given the finite number of these object it should be fairly unlikely to spot such an object in space near to the nucleus. Of course it can be modeled whether it would be plausible that such an object would be intact after (re-)impact. Finding two boulders that possess fractures that match would indicate that an object was broken on impact. It does not appear likely (?) that boulders measuring several metres across are original ejecta (too heavy). The density, chemical and mechanical properties of such hail has to be hypothesized before making any calculations on the likelihood of surviving impact. I find the lighter colour supporting of a crystalline nature, also the multifacetted surface resembles hail found on earth. Well I don’t know….what do You think?

Hailstone is the term referring to single object, while “hail” is the precipitation type..on earth hailstones form as agglomerates of H2O crystals (snow) that have passed through a phase of heating to obtain a “sticky” wet surface. In space “hailstone” might form when ejected icecrystals agglomerate and form irregular megacrystals by incorporating gas-phase ejecta. The low gravity will contribute to this, by allowing time to grow before reimpacting….Is this at all plausible? Anyone? Some interesting consequences wil be the likely sublimation of some of these proposed hailstone as we approach sun. Well, I’ll be watching…when given the opportunity.

Hi logan
Yes on electrical AF and RF signals i do fft with my instruments but not on 2D image information, but i get your point, there is a pattern in the surface that sticks out clearly. I supose some image processors can do some fft tricks.

Comeatstalker, i also made the computation of the mass when i saw it and calculated the equivalent force and it was the same but x1000, that ia a mass of 16000 tons and thus 260 kg there, possible to lift between 10 people which is impressive as well

Now since I do not have stereo coverage here I resorted to another computer vision technique called Shape From Shading. I make the assumption that the surface has constant albedo. Then any difference in value must be the result of the surface leaning towards or away from the sun.

Armed with that knowledge and with a fair sprinkling of Dragons Tooth in the Full Moon Light I can conjure up a surface.

Your great work shows that Cheops is not a spherical boulder, but is more in the shape of a shard and clearly shows why the dust has collected where it has. The appearance is also more homogeneous in this view with little evidence of it being a conglomerate type material or porous. It also shows better how at some time in the past it has toppled over the edge of a shallow buried crater, its pointing “downward”. If it has moved, it rules out the possibility it is solid material piercing the surface from below.

The nearby “rocks” visually look as if they have rolled down from the adjacent cliffs, but Cheops is too big and from its current shape looks unlikely to have rolled anywhere. It might be external impact debris from an impact elsewhere on the comet, where the composition of material is different and hence the rate of “erosion” is different from the surrounding area.

When it first landed its surface would have been “hot” and the pressure of its impact melted a small area of the surface enabling it to slide. That thin liquid layer would soon freeze leaving a lighter coloured trail of fresh ices that can still be seen through the later deposited regolith, as a lighter trail. A 20,000+ tonnes, (assuming some of it has sublimated away since), piece of hard, icy material should have left a large dent in the surface. At the end of the trench there is a darker depression of similar, but larger outline to Cheops. Without knowing gravity vectors and the elevation changes, its still difficult to say why it moved in the direction it appears to have done.

Visual evidence and intuition are of little help in sorting out what is and has gone on during 67P’s evolution. Those that know the numbers will hopefully be able to explain it all to us at some future date. Somehow I doubt it though.

The OSIRIS team did interpolate the original “native” resolution up by roughly a factor of two before sending us the picture to release here. That means that effective pixel size in the image shown here is about 25cm/pixel, as opposed the original 50cm/pixel. Thus the region shown in this image covers about 390 x 267 metres.

This interpolation has led to some checkerboard artefacts visible in the background if you zoom in closely. But it should not have affected the resolution, or very little at most.

The origin image is a 4 Mpix image that we received a third from, then its blown up a factor 2 by resizing and finally compressed with jpeg to a tiny file about 80kB, adding a bit blurr and chessboards is a result of this. Are there really so many secrets in this original image that it is worthwhile to cover them up? It could be done the other way around and stack 4 to 8 images of the same object do some fft and deliver a detail resolution that is hamburger size

Very, very impressive stuff so far. This mission is showing what space exploration is all about and can achieve with our current technologies. If this is only the start it promises for the future !
I really hope Philae has a successful landing and can show and learn us from up close what a comet is and how it behaves. Top notch work by the whole Rosetta team, ESA and NASA and whoever made this possible.

(quote)
” .. Site J offers the minimum risk to the lander in comparison to the other
candidate sites, and is also scientifically interesting, with signs of
activity nearby. At Site J, the majority of slopes are less than 30º relative
to the local vertical, reducing the chances of Philae toppling over .. ”
– – –

Hi ESA I would be more than glad if you were right.

The analysis of your “Boulder-Image” suggests the very opposite !

Analyzing the surface NW of the ‘UNIQUE BOULDER’ shows a dramatically
rough terrain.
It looks like the ENTIRE surface is painted with boulders in the (<1) to 5
metres size.

Provided Philae makes it down (<5%) there is an enourmous chance the vehicle
might be shaded from the sun by near-by or even 'next to it' boulders.

I find the whole discussion about whether this thing is actually a boulder (i.e. a pretty big lump of rock) or merely a “boulder-like” chunk of (name the preferred substance according to your pet theory) completely surreal. If we’d been shown this picture without being told it was lying around on Comet 67P/C-G, then NOBODY would have thought to even ASK the question! Everyone would have immediately reacted “It’s a rock lying around on Earth/on Mars”

@Thomas That is not quite true. If I saw a pitch black object such as Cheops and was not told where the image was taken, I would say coal. Black rocks are rare, and most black things we can have images of have some connection with life eg charcoal, soot, etc. it is one thing to question any number of premises of prevailing theories of comets, but ignoring the albedo, and very convincing mathematics/physics in calculations of the density of the comet is akin to denial of facts. Which part of the calculation of density do you think is wrong?

A mean figure for the density of sedimentary rock is about 2.5g/cm^3. This is just over six times the density figure revealed by ESA of 0.4g/cm^3. If the surface of 67P is indeed made of rock, then roughly 85% of it must be empty space. In effect its hollow. The first large lump of material travelling at normal type space speeds would smash it to bits.

In fact using the figures given in the earlier blog post, the thickness of a sedimentary rock layer covering the whole surface of the comet would be around 50 – 55m thick. For igneous or metamorphic rocks with higher densities it would be even less.

“Black rocks are rare, and most black things we can have images of have some connection with life eg charcoal, soot, etc.”

I don’t understand your point here: “connection with life” has never been part of the discussion or perhaps I’ve missed something.

The “black things” you take as examples (“charcoal, soot, etc.”) down here on Earth are invariably the result of carbonization through burning. I don’t see why things should be that different up there on Comet 67P/C-G: the (extreme) heat which has blackened the entire surface of the comet (exactly like that of all the other comet nuclei we have imaged so far) is produced by the permanent, ongoing electric discharge phenomena on rock of indeterminate composition which Rosetta is currently observing. When something containing carbon is heated, (whether wood, dirt, rock or whatever) it generally turns BLACK.

As for the “convincing mathematics/physics in calculations of the density of the comet” you invoke, that is a simple value judgment which you’re entitled to, which I respect, but which doesn’t “convince” me at all because I prefer to trust the evidence of my eyes. Sorry, I will never believe in rocks which have less than half the density of water.

Since we’ve been invited to believe that this lump of visibly stratified rock would bob up and down on any terrestrial ocean in the manner of a champagne cork, I got interested in how the “density” of small, faraway objects like asteroids or comet nuclei is actually “calculated”. I found this highly enlightening essay on Wikipedia: http://en.wikipedia.org/wiki/Standard_asteroid_physical_characteristics
with quotes like:
““Apart from a few asteroids whose densities have been investigated, one has to resort to enlightened guesswork.”
“For many asteroids a value of ρ~2 g/cm3 has been assumed.”

What’s the methodology? Hardly inspires confidence, does it?

So have I got this right? By the same process of “enlightened guesswork”, presumably comet nuclei (about which we know much less, even, than we do about asteroids…) are ASSUMED to have less than half the density of water since, according to standard theory, they are supposed to be “dirty snowballs” or “fluffy rubble-piles” (take your pick). It’s a totally circular form of reasoning, with the numerical result obtained depending entirely on the initial ASSUMPTIONS. In short, just GIGO…. Not much different, in fact, from Einstein’s admitted “blunder” in introducing the notorious “cosmological constant” into his theory of general relativity….

You can believe whatever your theory dictates. Personally, I only believe what my eyes are telling me. Rosetta is escorting a lump of rock. So its “assumed” mass/density is necessarily very inaccurate “guesswork”.

@Thomas, “invariably the result of carbonization through burning”. Burning here wil mean “heating” as “combustion” does not turn out the black carbon, whereas incomplete combustion does, due to lack of oxygen. Besides it is definitly not the case that ” black” requires high temperatures: sulphuric acid extracts water from sugar and leaves the black carbon. Radiation can do similar things, high temperatures not required. I think your “welders approach” needs the aid of some other points of view, as far as you are in fact in search for some applicable insight. I would advise you to recognise the reality of gravity. Doing math with gravity got Rosetta to where it is. Also I suggest that you generally appreciate observations that are not strictly visual: the spectra that reveal hydroxyl in comets coma are real, even if they may appear incomprehensible.

@ Jacob Nielsen, “Burning here will mean “heating” as “combustion” does not turn out the black carbon, whereas incomplete combustion does, due to lack of oxygen”.

The term “heating” suits me fine (I never referred to “combustion”). That’s precisely what I’m talking about. Like the heating that takes place at the base of a welder’s arc. When things are heated and there’s some free carbon around (and a limited supply of oxygen), it gives things a black, burnt appearance (like that of “charcoal”).

“Besides it is definitly not the case that ” black” requires high temperatures: sulphuric acid extracts water from sugar and leaves the black carbon. Radiation can do similar things, high temperatures not required.”

What’s the relevance of this statement in the context of our current discussion about the charcoal-black surface of Comet 67P/C-P? We know that. And so what?

Sorry but you quoted me out of context in the first place. I never claimed that burning/heating was the ONLY way of turning things black. It’s what we call a sufficient but not a necessary condition. In saying that “The “black things” you take as examples (“charcoal, soot, etc.”) down here on Earth are invariably the result of carbonization through burning”, I was simply responding to YOUR examples of ““charcoal, soot, etc.” and nothing else. I maintain my statement. They are.

Maybe the precise blackening process involved at the surface of the comet is completely different, we don’t know yet. After all, thanks to ROSETTA, we’re just at the beginning of genuine empirical, (rather than mere THEORETICAL) research into the nature of comet nuclei. There are no doubt other possible processes but they all seem to involve heat which is extreme enough to produce molten rock. For example, concerning the black appearance of most forms of lava down here on Earth, I just found this on the US government volcanowatch sitehttp://hvo.wr.usgs.gov/volcanowatch/archive/2000/00_10_19.html :

“When molten lava chills quickly, a thin, glassy rind forms on the exposed surfaces because crystallization of the minerals in the melt could not take place in such a short time. This layer of glass provides the iridescent luster to the rock. When crystallization occurs, the first minerals to crystallize are mafic minerals, which are dark-colored and contain abundant iron and magnesium. Rocks that cool quickly, especially the outer layers of a flow, are primarily composed of glass particles and tiny mafic minerals. This is why the outer surface of a flow is black.”

And so that there are no other misunderstandings, let me state right now that I KNOW that there are no volcanoes on 67P/C-G…! I’m only talking about analogous extreme heat conditions such as those produced by electric discharge phenomena.

@ Jacob Nielsen, “ I would advise you to recognise the reality of gravity. Doing math with gravity got Rosetta to where it is.”

Thanks for the advice…. Actually, I’m forced to recognize “the reality of gravity” like everyone else each time something slips out of my hand and crashes to the floor (and sometimes breaks…).

But woah! Hold your horses!
If “Rosetta [got] to where it is”, it is thanks to the EMPIRICAL knowledge we have acquired about the gravitational attraction exerted by the LARGER bodies in the solar system (planets, moons and some of the larger asteroids) via direct observation of their trajectories and the way those trajectories are occasionally modified by the attraction they exert on each other. (Even if we know absolutely nothing about what “gravity” actually is – but that doesn’t matter since our pragmatic calculations work for all practical purposes). It is thanks to this empirical knowledge that, crucially, Rosetta was able to take full advantage of the “gravity boost” it got several times from Earth and once from our nearest neighbor Mars to help it meet up with Comet 67P/C-G.

The fact that Rosetta has got to within a few miles/kilometers of 67P/C-G thus has absolutely NOTHING to do with the vanishingly small gravitational attraction of the comet itself.

Since it arrived in early August, Rosetta has been flown at varying distances from the comet by rule of thumb, on the basis of the local gravitational conditions observed on site (necessarily complex given the highly irregular shape of the nucleus, a very unpleasant surprise which was only discovered on arrival…), using the burn-times required each time. It was high time for a lot of very efficient improvisation! Hat’s off, again, to all the team-members involved.

The relevant data is logically even more “top secret” than the images which have been grudgingly distilled to us, as everyone has noticed….

@Thomas once a probe is orbiting a body, it’s mass can be very accurately be measured by the orbit of the probe. The volume can be measured by the dimensions and shape. The biggest uncertainty in the order of say 1% is the unimaged shadowed part. The guesses you refer to are for remotely sensed or flyby objects.
Returning to the nature of the substance on the surface : If you took a photo of a coal seam on earth, it would look like rock until you were told that the brighter areas were due to the flash or other lighting, then you would realise it was coal, which by the way, does float as opposed to rock. It only looks more like rock and less like coal due to the lighting, really!
If you said that comets were chunks off a planet sized coal seam, it would fit the visual evidence way better than barbecued sedimentary rock. Rock has little carbon in it in the first place, and when zapped would link up with the oxygen from say the hydroxyl ions or whatever.
I also want the Whipple model consigned to the dustbin of history, but it doesn’t help when you deny accurately calculated facts based on visual evidence.

@Marco. My answer regarding gravity is much the same as to Jacob N, just above. Ditto for the blackness of the comet.

I would simply add that I don’t deny any sort of “visual evidence”, quite to the contrary. That’s the whole point I’m trying to make. The black aspect of the nucleus is clearly due to some form of carbon, whether of organic or inorganic origin, and has been produced in conditions of extreme heat.

I simply think that the most coherent explanation for this heat is electric discharge phenomena as the nucleus undergoes increasing electrical stress as it approaches the Sun.

Enough images have been released with true time stamps and distances that any thorough and obsessive graduate physicist with plenty of time can verify the calculations made. At a distance of 30km the shape of the comet has a negligible effect on the correct calculation of total comet mass from the speed and distance of orbit which themselves can be calculated and verified even with just the information we have been given. Ie from the images and verifiable date/time/distance data. We can verify any data we have been given by the images themselves, given enough patience and scientific expertise. Just to fly Rosetta, the team would need to know all this to model it’s position given they don’t have real time data, they have to know where it is going to be by the time signals go back and forth. What you are really saying is that they are lying about this data to keep to a Whipple model. Not completely crazy but that ship has sailed. “Snow” doesn’t have the integrity to shape to a bilobe structure, crust or no crust. The low density, shape and colour needs to be explained with a model that makes all these things likely, and that fits the evidence from stardust and deep impact also. A model that predicts facts not in evidence yet but that will be in the future, is a good test of the model also. I prefer to think that the carbon is from organic living material, thus more like a crude oil gel ie hydrocarbons, rather than charcoal, soot.

@Marco, I confess to not sharing your apparent faith in “the information we have been given”, whether it be the scant data about Rosetta’s burn-times and “orbits” (which of course they are not, in the common acceptance of the term), or even the theoretical underpinnings of the required calculations.

To quote the beginning (with my capitals for emphasis):
“Mysteriously, four spacecraft that flew past the Earth have each displayed UNEXPECTED ANOMALIES IN THEIR MOTIONS.
These newfound enigmas join the so-called “Pioneer anomaly” as hints that UNEXPLAINED FORCES MAY APPEAR TO ACT ON SPACECRAFT
A decade ago, AFTER RIGOROUS ANALYSES, anomalies were seen with the identical Pioneer 10 and 11 spacecraft as they hurtled out of the solar system. Both seemed to experience a TINY BUT UNEXPLAINED constant acceleration toward the sun.
A host of explanations have been bandied about for the Pioneer anomaly. At times these are rooted in conventional science — perhaps leaks from the spacecraft have affected their trajectories. At times these are rooted in more speculative physics — MAYBE THE LAW OF GRAVITY ITSELF NEEDS TO BE MODIFIED.
Now Jet Propulsion Laboratory astronomer John Anderson and his colleagues — who originally helped uncover the Pioneer anomaly — have discovered that four spacecraft each raced either a tiny bit faster or slower than EXPECTED when they flew past the Earth en route to other parts of the solar system.
‘Humble and perplexed’
The researchers looked at five deep-space probes — Galileo to Jupiter, the NEAR mission to the asteroid Eros, the Rosetta probe to a comet, Cassini to Saturn, and the MESSENGER craft to Mercury. Each spacecraft flew past our planet to either gain or lose orbital energy in their quests to reach their eventual targets. (Galileo made two flybys.)
IN FIVE OF THE SIX FLYBYS, THE SCIENTISTS HAVE CONFIRMED ANOMALIES.
“I AM FEELING BOTH HUMBLE AND PERPLEXED by this,” said Anderson, who is now working as a retiree. “THERE IS SOMETHING VERY STRANGE GOING ON WITH SPACECRAFT MOTIONS. WE HAVE NO CONVINCING EXPLANATION FOR EITHER THE PIONEER ANOMALY OR THE FLYBY ANOMALY.”

Now these anomalies (even if they are “tiny”) were detected by standard Doppler measurements carried out in our backyard with respect to the gravitational attraction exerted by colossal, normally “well-known” masses. The margin for error is necessarily huge when the same standard Doppler measurement techniques are being used to attempt to determine the mass, hence the density, of what is, in comparison, this tiny speck of dust/grain of sand/rock which Rosetta is attempting to “weigh”. The Doppler differences observed by ROSETTA are necessarily too tiny to enable us to draw such confident conclusions as those so far advanced. I don’t see why we should have greater confidence in the theoretical underpinnings of the so-called “findings” than the eminent sources I have quoted seem to have.

I maintain that the “density” values so far given for this lump of visibly stratified rock are most likely derived from prior assumptions of what comet nuclei are supposed to be and that the actual measurements taken involve such a wide margin of error that anyone can pick and choose their own preferred value, according to the model they are defending.

I’m not saying at all that the Rosetta team are “lying”, just that they’re not as yet disclosing all the relevant information. They are actually making no bones about it, in fact….

To put it another way, I agree with your “negative” assertion- ie. that the Whipple model is demonstrably wrong and should be abandoned. I don’t agree with your “positive” assertion regarding rock and electric phenomena. The density calculation is neither here nor there, as a low density doesn’t help the Whipple model given the shape of the comet and it’s features.
As far as data is concerned, you can take the density calculation to the bank, however.
The problem with science, is not the physics, but not being able to drop a theory or model because there is no clear consensus amongst the peer group on what should replace it. I would be happy enough to have a multitude of options on what would replace the model, without any being the “agreed to” model

That depends on how you define it Cosmo.
Of the full frame there is one third of the image presented.
Then there is no information of the filter used, or if a filter is used and as its optical bandwith is from UV to NIR so if that has a weight of 0.3 then about 10% is delivered.

On the other side thats a lot more then we ever got before and possibly we might get some additional pictures to add to our jigsaw.

If you count the Pixels we got a bit extra as at first the image had 4MPx then someone cut two third away, the rest was scaled up a factor about two quadrupling the amount of pixels to about a total to be presented is 4*2*2/3~5.33 MPx, then before presenting the result it was stripped down to 80 or so kiloByte by jpg compressing leaving some of the original information there but in a blurred and disordered shape. Still what remains is quite nice to look at in an artistic kind of view. ESA is protecting us from getting our minds to stressed, you must not tell a child the full truth all at once, better wait till it grows up and finds it out by itself anyway:o

Just wanted to direct your the attention to the fact that carbon black has the lowest thermal conductivity of known substances. The lowest it gets, apart from vaccum ( which is not a substance). The surface of a comet is surely composed of something very close to pure carbon black. Now if I had to fly a block of ice through the corona of the sun I would know no better way to preserve it (or at least some of it) than to cover it in a blanket of carbon black.

I believe it was in a BBC News interview done at the time of the arrival. I have not checked,but it was also mentioned in the science briefing at ESA on the afternoon of the arrival. A similar description, including the analogy to printer toner was made by the NASA scientists in their lecture earlier this week. The surface layer was “speculated” (I know unusual that) to be made up of dry fine dust mixed with hydrocarbons/organics, possibly akin to tar, to make a very dark porous layer.A similar speculation appeared in an ESA video as well, the video update on 23/9/2014, though dust was not mentioned.

In the NASA lecture a list of possible components in that dust that they were looking for with the ALICE instrument, included Silica, Aluminium Oxide, Carbon, possibly in the form of diamond, and various other rock minerals, in other words, ceramic materials. I would imagine the lecture is available on their website, nasa.gov

Strangely the archived video referred to on this page misses the first few minutes of the original show, which included mention of the nature of the surface. Methinks someone gave away more than they should have.

An inner snowball was preventing from melting in these extreme conditions by a carbon crust? Is this the sort of contortion standard theory has to resort to, to keep itself intact, just like the hypothesized Lovejoy “snowball”?

It is hard to understand this event but the few hours in the multi mega kelvin plasma in the Corona-sphere would be my least concern. The tidal force is a bit worse and so is the hard radiation. I suppose whatever the substance of this comet is made of it got a lot of its surface blasted away. The inside got penetrated with a broad spectrum of electromagnetic waves. It must be a nickel iron core to withstand this treatment, a comet like the 67pcg would scatter to pieces long before it reached the corona due to tidal force. Thermal conductivity in solids is not very fast in transfer rate and the hot plasma in the corona is far less dense then solids. If the surface is of ice or carbon or steel makes not a lot of differeny as it is stripped of layer by layer due to the plasma and the comets huge speed trough it. There are some pictures of comets that are a bit more fragile and scatter apart when getting close to the sun or to Jupiter.

@ Cometstalker, you say “… a comet like the 67pcg would scatter to pieces long before it reached the corona due to tidal force”.

Sorry?

-What do you know about the exact nature of 67P/C-G to allow you to make this statement?

-Why should it behave differently from Comet Lovejoy, which survived its hour-long flight through the Sun’s 2 million K corona?

-What known physical property enables you to surmise about Comet Lovejoy that “It must be a nickel iron core to withstand this treatment » (flying through the Sun’s corona)? Nickel and iron both melt at around 1,500°C – less than 0.1% of the temperature of the Sun’s corona.

I thought all comets were supposed to share much the same properties in being just “dirty snowballs”. Now that we’ve seen with our own eyes that 67P/C-G certainly isn’t, does any ad hoc explanation for individual comets go?

When, for example, the 21 fragments of Comet Shoemaker-Levy 9 each exploded on contact with Jupiter’s atmosphere back in 1994, may we legitimately assume they were packed individually with TNT? Or when the most spectacular comet of the 20th Century, Hale-Bopp, was still able to display a respectable coma in the near absolute zero temperatures beyond the orbit of Neptune as recently as 2011, must we conclude that during its perihelion in 1997 it had ingeniously managed to store up the Sun’s thermal energy in some kind of internal ice-batteries?)

Personally, I prefer a less piecemeal, more holistic approach to our interpretation of comet observations, an approach which assumes that there is no need to be forced by every new observation to patch up the standard theory (or everyone’s pet theory) and that all these comets do indeed share a common property: that of being electrically charged lumps of rock on eccentric orbits with respect to the Sun. (They are electrically charged precisely BECAUSE they are on eccentric orbits with respect to the Sun). They thus have to deal with the (electrical) stress of having to “fly headlong” into the stream of ions which standard theory euphemistically calls the “solar wind” (“wind” in space?) and which, in any other context, would be called an electric current. The electric charge imbalance thus generated in turn unleashes the observed DIScharge phenomena which is “Nature’s” way of overcoming the crisis. (A bit like you and me, in fact: it never does any harm to blow your top occasionally, when the stress gets too hard to handle by more conventional means…).

The actual value of the charge imbalance depends partly on scientifically well-known factors such as the shape and speed of the orbit but also on currently less well-understood (and hence less predictable) factors such as the strength and direction of the “solar wind” with respect to the comet at a given time and place in its orbit.

This holistic approach provides a coherent explanation to all the observations we have acquired of comets, comet nuclei and cometary behaviour over the past couple of decades, including the “stardust” samples from Comet Wild 2, virtually all created in conditions of extreme heat, and the double flash observed on the impact of the “Deep Impact” projectile on Comet Tempel 1.

There is no need to constantly put patches on the “electric universe” model. It rests on a single tiny, but all-powerful, difference between a plus and a minus which makes ALL the difference, from the infinitely small up to the infinitely great which we can hardly even start to get our minds round.

The matter in the cloud our solar-system once was created of, was separated in a chain of processes. 99% or so ended up in our sun the rest is a bit colder and far out it is cryogenic. Our outer bigger planets seem to contain little minerals and we are supposed to think comets that are very far out have a lot of water. Actually i think that once the sun ignited its solar wind stripped of a lot of material in the planetary disc and this mixed up with the outer end of the disc where comets are suposed to be created. Most likely this is just an idea and has none or very little substance of reality. Observations with instruments of distant passive objects are not accurate and leads to a lot of misinterpretation. The comets delivered to us for closer observation has not given us a lot of essential information.
Now we are at this specific comet and the only thing we know so far for sure is that it does not fit very well to a general picture of what comets in general are suposed to be like.

For sure is that its average density is really low, its as black as can get, it is dry and if there is water then it is inside and mixed up with other stuff and frozen. It is streaming out material from its neck region.

So far the real hit was the low average density and to me this indicates that this is a huge lump of old dust in a way very porous and it has some soaked up water, amonia, alkohols and hydro carbons. The few lighter gases left, like Helium are rare and there is no way to tell if this is from the original matter or sun wind.
I don’t think that there are any general comets around at all. To me it seems that the leftovers of the solar system creation process are nothing but leftovers and its vareity is even greater then what is found in a trash can.

It is exiting to find some clues about this comet but it will tell us nothing about how life on earth started or how some of it became persons observing the comet.
It will only tell us a lot of how this particular comet is doing right now and possibly some of its very recent history. Great is that some RNA, DNA after a while is able to do this kind of job and receive world wide web congratulations.

Its hard not to agree with you there Comeatstalker. As was pointed out elsewhere, we thought we knew all about asteroids and then some astronomer goes and finds a couple with tails like comets. Comets and asteroids are tiny compared to planets. There are millions of them flying about the solar system, perhaps billions if you include the Oort cloud. The results from half a dozen examples are therefore statistically virtually irrelevant when trying to come up with a theory for all comets or all asteroids.

One of the stated aims of the Rosetta mission is to get a better understanding of how the solar system evolved. The clear implication being that current theories are at best approximate and realistically incomplete. The solar wind has been ever present since the sun first established itself as a star, along with a huge magnetic field. The interactions between the charged particles of the solar wind, statically charged dust /molecules and this magnetic field are only now starting to be investigated. There are large energies and forces involved which must have had significant effects on the evolution of the Solar System.

Rosetta won’t answer these questions, or many others that are claimed for it, but having got there and seen it, just finding out everything we can about this one amazing object is enough for me.

Hello all!
My name is J and I’m a freshman at a local Jr. college in California. I am doing an oral report on this mission and I was looking for any advice I can get, since my science/ astronomy knowledge is pretty non-existent. I am however a fantastic public speaker and if given a little extra knowledge from passionate people like yourselves, there’s no doubt in my mind that I won’t be able to knock it out of the park. They say C’s get degrees, but I want an A!!! Help! thanks all
fuzzywuzzy383@gm***
-J

Hi Jayson, for a quick way to get up to speed on Rosetta then I point you to these resources:
-Video: http://www.esa.int/spaceinvideos/Videos/2014/01/Rosetta_the_story_so_far (covers launch to pre-waking up from deep space hibernation in jan 2014);
-http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_factsheet [simple summary fact sheet of the mission]

And then browse the news items/follow links to more videos/images at esa.int/rosetta for the latest news and more in depth articles (also of course this blog for more discussion!)

If you have any specific questions about the mission, then you can post it here and someone will answer!
Good luck for your report!

Maybe try imagining it has not been around since the beginning of te solar system, Then try to think of some reasons why it looks like it does.
All of the Comets seen close up present problems for the dirty snowball idea and then the belief that its been around since creation idea also looks very shaky.
This means the door is open, we are free to postulate what is happening why and when, its a lot more interesting than continually looking for non existant ice, and as the boulders dont have any either, how deep are you willing to postulate that the depth of the ice is?, before you look again.
Science should allow us to question, forcing data to fit any model is always going to be wrong, lets use our imaginations.
The formation of our solar system theory looks shot to pieces anyway, Its always been a bit of a strectch as a model of creation.
So if the opinion about creation is not really belived then why should the theory of the comets life be any more likely to be right.
Just look at the surface. if you believe all the craters are impacts then how on earth did it last for billions of years, surely even without loosing material near the sun it would of been blasted to bits in the outer regions by now.

The scale is just fine and even if the accuracy presented is a bit fuzzy its good enough for most of us to get an idea of it. If ESA has a problem then it is not the size of the comet but rather the amount of data that they receive and the fact that they still have not one single clue of just about anything of essence to present. The scientist at ESA are apparently far far away from the goal and still stumbling along on their marathon leap. So far Philae has not landed yet and once it has it will turn into a triathlon struggle for these poor overwhelmed scientist already on their edge of capacity. This time it is going to be quite funny to watch the walls of Jeriko collapsing.

I like very much read the posts and the comments. i learn a lot. Now i have a question than somebody can explain to me. I suppose the surface of the comet must be like porous compacted “grounds of coffee” that allows the volatile compouns scape slowly to the space (no violent jets, only from time to time soft bursts that expose fresh surface that we can see as sudden jets from our far far away point of view) how the shade can “protec” the surface to avoid the outgasing and form the feature that we can see around the boulder?

Hi Ruben. If Cheops is made up mainly of ices because, as we can see, the amount of “dust” on its surface is not very extensive, those ices will sublimate more quickly than the area surrounding it. The plain around Cheops does have a thicker layer of insulating material so its rate of sublimation is likely to be less. Over time Cheops will reduce in size, therefore exposing the surface to deposition of dust around its edge. Close to Cheops the dust layer is thin, as you move away from it gets thicker as dust has had more time to build up. This gives the shallow bowl we see surrounding Cheops.

Somewhere in space Thor, the god of lightning, or maybe Hefaistos, the ancient god of electrode welding, hit loose the tip of a massive mountain, hit i so hard it got all zapped and confused and forgot to bring along some gravity. That’s how 67p came along.
In the case of comets, the jury is out, and we get to play!

The vector of gravity on this place is only of concern to the bombing trajectory of Philae, once its close to the surface it will align itself the best it can and from then on the inertia is all that counts, the energy of the impact will be absorbed the best way possible with an electro mechanic brake system. Once hanging on with its gadgets to the comet it doesn’t matter if it hangs on a wall or from the roof. The gravity constant is so low that a couple of ants can handle the force the lander weights on the comet.
I tried to find a Lagrange point on this comet where gravity and the centripetal acceleration could make things to localy “float” but if the comet has a uniform density these points are always under the surface in the neck region. If it has one or more dense nickel iron cores in worst positions it is possible to get lagrance points outside and close to the surface. The geostationary orbit for a spherical mass of this size is about 3250 meter radius thats pretty close to the surface. Check the calculation by using the online calculators for gravity force and centrifugal force, use the comet mass 1e13 kg and the orbiter mass 100 kg the orbiter time must match the comets spin about 12.4 hours per turn and set the radius to 3230 meters and check if the forces are the same ammount about a hand full of milli Newtons.
With a bit of strain it is possibly to simulate the event with two bodies orbiting each other one weights 7.5e12kg and the other 2.5e12kg the period of rotation still 12.4 hours.
The radius is the clue of how the comet stays in shape.
The team of rosetta know a lot more about the comets graviton uniformity but are sitting on this information.

Heaps of stuff have a tendency to lump together, the more complex the compositions of its contents the more possibilities to create shapes. It started with a pair of simple atoms H and He and i don’t know if it ends with complex molecules like DNA and its varieties. Comets are not very special and to my belief just the trash remaining from the creation of our simple solar system.

But I do think that the “anomalies” observed may indeed show that there is an electromagnetic component to what we call “gravity”, the magnitude of which is dependent on local (rather than constant, universal) factors. Which in turn would have a direct bearing on the calculation of the density of 67P/C-G.

Its always the same story when scientist are baffled with their measurements. The first thing they do is to try to find an explanation that explains their results like; there must be dark energy or dark matter or gravity is not as it should or neutrinos can travel faster than light. Only a lot later they admit that the measurements had errors and never will they learn their lessons.